A fluid coupling apparatus of the above kind is for example described in the specification of U.S. Pat. No. 4 944 374, but with the major difference that that known coupling apparatus does not have a torsion damper. Such a fluid coupling is more particularly intended for a motor vehicle, and more precisely it is adapted to be interposed between a driving shaft, typically the crankshaft of the vehicle engine, and a driven shaft which is typically the input shaft of a gearbox. The fluid coupling apparatus described includes a simple, resiliently deformable coupling plate which is interposed between the above mentioned coupling element and the hub of the turbine wheel of a torque convertor. The deformation of the coupling plate in the axial direction enables the coupling piston (which is provided with a friction liner) to grippingly engage the casing of the torque convertor. The effect of this is to take the torque convertor out of use in certain configurations of the transmission. For this purpose, the coupling plate includes through apertures in the form of buttonhole-shaped slots, which define at its periphery two sets of resiliently axially deformable tongues, namely a first tongue which is adapted to transmit the torque in propulsive operation, that is to say when the engine is effectively driving the vehicle; and a second set of tongues which is adapted to transmit the torque when the vehicle is operating in a regenerative mode, that is to say when the inertia of the moving vehicle itself is tending to drive, or brake, the engine. The tongues are here joined at their ends to the transmitting element constituted by the coupling piston. When the latter comes into contact, under a hydraulic control pressure, with the wall of the casing, the resulting locking or gripping action enables the turbine wheel to be driven directly by the casing itself. The turbine wheel is coupled to the driven shaft, e.g. the gearbox input shaft, while the casing is coupled in rotation to a driving shaft, e.g. the engine crankshaft.
In certain applications it can be desirable to complement the locking clutch with a torsion damper, such as a conventional damper of the kind comprising helical springs arranged circumferentially between two components which are rotatable relative to each other. In that case, a coupling plate of the kind described above forms part of the torsion damper itself, but it is necessary that the whole of this torsion damper should remain under all circumstances perfectly perpendicular to the axis of rotation of the apparatus, that is to say the common axis of the two shafts mentioned above. However, when the coupling element is displaced, the tongues can distort in an uncontrolled manner under the effect of too large a torque, and the result of this can be to upset the perpendicular relationship between the torsion damper and the general axis of rotation.